1. Field of the Invention
The present invention relates generally to optical communication systems and, particularly to small form factor high-speed optical transceivers with high sensitivity receivers.
2. Discussion of Related Art
There is currently a demand for high-speed, long-distance, cost-effective, and highly integrated optical transceiver systems. Therefore, optical receiver systems that are high performance, low cost, and are of small size are desired. For short reach and medium distance transmission, a PIN photo detector based receiver can be used. For long reach transceivers (e.g., for transmission over tens of kilometers), however, high sensitivity receivers are required to compensate for the optical signal loss due to long optical fibers. In long reach systems, then, an avalanche photodiode (APD) can be utilized instead of a PIN photodiode because of its higher sensitivity and lower noise characteristics.
However, the active area of a high-speed APD is generally limited in size. A typical active area diameter can be about 35 μm for a 2.5 Gb/s APD chip. These APD chips can be obtained from Mitsubishi or other manufacturers and generally include an avalanche photodiode detector and a transimpedance amplifier. In order to focus light from the signal-carrying optical fiber onto the small active area of the APD, high quality optics and tight alignment tolerances are typically used. Typically, ashperical lenses can be used to reduce optical aberrations in order to form a small spot size on the active area of the photodetector. High precision laser welding processes are often used to manufacture the receiver optical subassembly (ROSA) with a small active area APD because of the tight optical alignment tolerances. The cost of the APD-based ROSA, then, can be very high due to the intrinsic high cost of the APD, as well as the more expensive optical lenses required in the lens system and very involved assembly process.
In addition, the APD-based ROSA can be used in long reach transceivers, where requirements for low back reflection leads to a need for high return loss characteristics. In some systems, anti-reflection (AR) coatings on optical components, including optical fiber ends, lenses, and detector chip surfaces, can be used to minimize the back reflection. Therefore, there is a need for lower-cost, higher-performance optical subassemblies capable of receiving long reach optical signals.